1. Field of the Invention
This invention generally relates to a communication control method, and in particular to a facsimile communication control method for controlling communication of image information between a facsimile transmitter and a facsimile receiver.
2. Description of the Prior Art
In facsimile communication, as the transmission rate of data increases, transmission time required for transmitting data decreases. However, since response speed is limited by the minimum scanning time of a facsimile receiver, i.e., recording speed of a recorder, there is an upper limit in transmission rate even if data are to be transmitted as band-compressed in the form of codes using a coding technique such as M.H. ( Modified Huffman ) coding system.
FIG. 1 illustrates in block diagram the main structure of a typical facsimile system which processes image information. As shown, the system is comprised of a transmitter section, which is shown as an upper half, and a receiver section, which is shown as a lower half. The transmitter section includes a reader 11 for reading an original to be transmitted, thereby converting optical image information into electrical image information which is comprised of binary data, an encoder 12 for encoding the binary image data supplied from the reader 11, a buffer memory 13 for temporarily storing data to be transmitted and a MODEM 14 for modulating the data to be transmitted. On the other hand, the receiver section includes a recorder 21 for recoding transmitted image information on a recording medium, a decoder 22 for decoding the coded data as transmitted from the transmitter, a buffer memory 23 for temporarily storing data and a MODEM 24 for demodulating the data received as transmitted from the transmitter. Also shown in FIG. 1 is "M.sub.T " indicating the capacity of the memory 13 and "M.sub.R " indicating the capacity of the memory 23.
The reader 11 scans an original to be transmitted thereby obtaining electrical image information in the form of the binary data, i.e., 0's and 1's, which is then supplied to the encoder 12 comprised, for example, of a microprocessor. The encoder 12 carries out coding of the binary image data according to the Modified Huffman system while controlling the operation of the buffer memory 13. In this instance, the encoder 12 carries out control such that encoded image data may be supplied to the MODEM 14 from the buffer memory 13 in accordance with data transfer clock pulses at the MODEM rate supplied from the MODEM 14.
At the receiver side, the encoded data received as transmitted from the transmitter is demodulated by the MODEM 24 and then thus demodulated data is temporarily stored into the buffer memory 23. The decoder 22 is, for example, comprised of a microprocessor and it functions such that the encoded data temporarily stored in the buffer memory 23 is decoded to recover the original image information in the form of the binary values, which is then supplied to the recorder 21.
In a facsimile communication system as shown in FIG. 1, image signal processing has been conventionally controlled to satisfy the relation of EQU V.sub.S &gt;T.sub.MIN ( 1)
where,
T.sub.MIN : minimum transmission time per line (sec/line ), and PA1 V.sub.S : reading speed of reader 11 ( sec/line ). PA1 V.sub.e : encoding speed per line ( sec/line ), and PA1 V.sub.m : MODEM rate (bps); EQU V.sub.d .gtoreq.V.sub.e ( 3) PA1 V.sub.d : decoding speed of decoder 22, and PA1 V.sub.e : encoding speed per line; and EQU V.sub.P .gtoreq.V.sub.S ( 4)
Moreover, the following relations must be satisfied. EQU V.sub.e .times.V.sub.m .gtoreq.T.sub.MIN (2)
where,
where,
where, V.sub.P : recording speed of recorder 21 ( sec/line ).
In prior art facsimile communication systems, control has been carried out such that the above four relations be satisfied.
Under the condition, in which the Modified Huffman coding system is used with constant MODEM rate V.sub.m, in order to shorten the transmission time T.sub.MIN in each of the relations (1) and (2) must be made smaller. In order to keep T.sub.MIN smaller, V.sub.S and V.sub.P in the above inequality equations (1) and (4) must be made larger. In other words, it is required to make the reading speed V.sub.S of the reader 11 faster, so does the recording speed V.sub.P of the recorder 21, which then would bring about other problems of making the system bulky and pushing up the cost.
With the reading speed V.sub.S and recording speed V.sub.P at constant, even if the encoding speed V.sub.e or decoding speed V.sub.d were increased, the overall transmission speed of the system would not be improved and there will be a fixed upper limit. Under the circumstances, even if transmission of data is carried out at a speed exceeding such an upper limit, it is useless because the receiver cannot respond. As a result, data transmission must be carried out with insertion of fillbits, which rather destroys the idea of shortening transmission time.
For example, in a facsimile system in which data is transmitted as band-compressed with a plurality of lines as a block, designating the time required to record one block of data by the receiver by T.sub.1 and the time required to transmit such a block of data by T.sub.2, it is often so controlled to hold the relation that T.sub.1 is equal to or larger than T.sub.2 with the insertion of appropriate number of dummy bits at the transmitter side. In this manner, transmission time T.sub.2 is required to be larger than recording time T.sub.1. For this reason, if the compression rate of data is high, a larger number of dummy bits are required to be transmitted, and, as a result, transmission time is very little reduced.
Instead of inserting an appropriate number of fillbits so as to keep the minimum transmission time per line as described above, another prior art approach is to provide a buffer memory between the decoder and demodulator MODEM of receiver to control the insertion of fillbits by sending a signal to the transmitter whenever the buffer memory of receiver is about to overflow, or, alternatively, by carrying out calculation at the transmitter side so as not to overflow the buffer memory of the receiver side. These prior art methods allow to reduce the total number of fillbits to be inserted, thereby permitting to shorten transmission time. However, difficulties will be encountered in practice if an attempt is made to carry out control so as not to overflow the buffer memory of receiver.
As described above, in the prior art methods, an attempt to increase reading speed V.sub.S or recording speed V.sub.P to shorten transmission time necessarily caused increases in device size as well as in cost. The prior art methods are also disadvantageous because a simple increase in encoding and/or decoding speeds, which can be realized relatively easily, does not lead to a reduction in transmission time.